Process for evaporating sodium sulfate solutions and recovering sodium sulfate therefrom



June 2, 1953 J. v. WISEMAN v 2,640,762v

PROCESS FOR EVAPORATING SODIUM SULFATE SOLUTIONS AND RECOVERING SODIUMSULFATE THEREFROM Original Filed NOV. 25, 1949 2 Sheets-Sheet 1INVENTOR. Jams: V. Ms'EMA/V ATTORNEYS Patented June 2, 1953 PROCESS FOREVAPORATING SODIUM SUL- FATE SOLUTIONS AND RECOVERING SO- DIUM SULFATETHEREFROM James V. Wiseman, Westend, Calif., assignor to West EndChemical Company, a corporation of California Original applicationNovember 25, 1949, Serial No. 129,393. Divided and this application July13, 1951, Serial No. 236,608

2 Claims. 1

This invention relates to apparatus for the concentration andevaporation of solutions of salts and which is particularly applicableto the production of anhydrous forms of salts having an invertedsolubility or a negative temperature co-efiicient of solubility curve,that is, the salt becomes less soluble as the solution temperature isincreased.

Preparation of anhydrous forms of salts such as sodium sulfatedecahydrate, sodium carbonate decahydrate and ferric sulfateseptahydrate presents special difficulties when attempted by evaporationusing heat transfer through a solid wall. Salts having a normalsolubility curve may readily be dehydrated by such means as amultiple-effect evaporator or other apparatus in which heat istransferred through a metallic wall to raise the temperature above theboiling point of the solution and thus to evaporate the liquid from thesolute. When this is attempted with salts such as those mentioned,however, the salt, being less soluble at the higher temperature directlyadjacent to the wall than at a somewhat lower temperature existing averyshort distance from the wall, deposits upon the heat transfer wall.The salt, not being an efiecti-ve conductor of heat, quickly decreasesthe efilciency of operation to such an extent as to make its costexcessive. This necessitates frequent shutdown of the apparatus andremoval of the sale scale or film on the wall, as by boiling out oractual hand scraping. Not only is a portion of the product lost in thisfashion, but the shut-down time and the expense of operation isincreased greatly.

There are other methods for recovering the anhydrous salts from theirhydrated form, but all have disadvantages. For example, sodium sulfatemay be salted out by the use of solid sodium chloride, but not only isapproximately 30% of the sodium sulfate lost, but the sodium chloride isnot recovered. The process accordingly depends upon a plentiful supplyof comparatively cheap solid sodium chloride.

I have found that difiicult concentrated ma-- terials, such as theaforementioned salts, which have negative temperature coeflicients ofsolubility, can be evaporated successfully if the liquid to beconcentrated is brought into contact with a relatively hot dry gas undersuch conditions that the gas is (a) cooled to the desired temperature ofthe eflluent liquid, (b) saturated with water at such temperature and(c) all surfaces confining the hot gas in the presence of the liquid aremaintained wet with sufliclent of a film of the liquid to prevent thesalt from precipitating from the liquid filmonto such surface. If theseconditions are observed, then the concentrating operation can becarried, on continuously, the water evaporated from the solution beingcarried off in the efiluent hot saturated gas stream.

The liquid removed from the evaporator is saturated with respect to thesalt and contains some of the salt crystals in suspension. Additionalsalt is recovered by adding, make-up hydrated salt at this point. In thecase of sodium sulfate decahydrate, this gives up its Water ofcrystallization above 32.5 C. so that the solution joins additionalwater and sodium sulfate, the solution being at a temperaturesubstantially above this transition temperature. The salt whichprecipitates is separated, the remaining solution then being returnedfor evaporative removal of the water added as water of crystallization.In this manner a continuous and trouble-free process can be practiced.

It is an object of this invention to provide an apparatus to evaporatethe water from the hydrated form of salts, and particularly one havingan inverted solubility curve, while substantially completely avoidingthe caking-up of the salt on the walls of the container in whichevaporation is carried out.

It is a further object of the invention to provide an apparatus fordehydrating salts by the application of heat directly to the solutionand not through the medium of solid wall.

A further and more specific object of the invention is to provide aneconomically feasible and practicable apparatus for dehydrating sodiumsulfate decahydrate and sodium carbonate decahydrate by the use of heatto evaporate the water therefrom.

A further and additional object of the invention is to provide apparatuscapable of extended periods of use in evaporating water from solutionsof salts in water without necessary shutdown for washing out or othercleaning.

Other objects of the invention will appear as the description proceeds.The invention will be particularly described as it has been applied todehydration of sodium sulfate decahydrate but this is only for purposesof illustration although the apparatus is particularly suited to-this.

In the drawing, Figure l is a side elevation partly in sectionillustrating somewhat schematically an evaporation construction, whileFigure 2 is a section taken through the line 22 in Figure 1. Figure 3 isa flow sheet showing the evapshell. Adjacent the upper end -44 of theshe'll is provided an annular bafile '46 having a central tube or pipe41 depending toward the lower end of the tubular shell 4 I. An outletfor combustion gases is provided in the side of the tubular shell 4|just below the annular bailie 46, this being shown as a conduit 48connected to the inlet of a blower 49. The lower end .of the tubularshell is preferably tapered inwardly as at 5| and a drain 55 and anoutlet 52 are provided, the outlet feeding directly into mixing tank 53.The 'mixing tank includes a central well 54 and a stirring device 56operating within the well. "Feed isintro'duced into the mixing tankthrough line 51 while the overflow from the tank is passed through line59 into a central well 63 in a'thickener orsettling tank 6!, from whichthe product is drawn off through line 62.

The overflow from the settlingtank is sent back-through line-66, pump G1and lineBB and is introduced into the evaporating chamber-in the spacebetweenthe central tube fl'and'the tubular vessel '4! at anupper levelby several spray noz- 21% 'll discharging into the space between thepipe 4'! and 'the'tubular shell AI and at a lower level, by severalspray nozzles "I2 discharging inwardly from the side of the shell 4| andonto the lower end of the :pipe to maintain this covered with a liquidfilm. In addition, a portion of the material in the mixer 'tank is takenoff through line T5 and returned by pump 11 through line 18 and header85 to discharge through tangential outlets 8i placed at equal distancesaround the tubular vessel 41 and closely adjacentto the annular baifie-46; material can be taken from the settling tank to line 76 if desired.

In operation, theburner was operated to discharge combustion gases near2,000" F. into the upper-portion of the tubular vessel. The tubularvessel '4! was 16 longand 3 in diameter. Bafiie 4B was placed a footfrom the upper end of the shell while pipe El was l0 long and 10"in-diamete-r. The return material supplied through the four tangentialoutlets 8i was discharged at such a rate that the spiraling materialheld by centrifugal force, built up in the interior of the vessel andstood on the annular baflie at approximately an angle of 45 andalong-thedotted line indicated at 69. The liquid flowed down over theinterior of the pipe 41, sufficient liquid'being introduced to maintainthe inner surface of the pipe 41 wet with liquid so that this surfacedid not become dry and so that it did not attain the temperature of thecombustion gases; to the sameend. nozzles 12 maintained the lower end ofthe pipe wet. All portions of the shell and pipe which could serve as aloci for salt crystallization are maintained wet with sufficient liquidso that heat transfer cannot occur except through a'liquid film to themetalsurface. Wetting the metal surfaces with sufilcient liquid preventsany heat transfer through the tube and so prevents the salt fromcrystallizing out uponthe surface of the tube. The gases which passupwardly between the tubular vessel 41 and the pipe 41 become saturatedwithwater issuing from'the blowerat abouti l'ild l";

Hat the rate of 40 gallons per minute.

Material from the settling tank is passed through line 62 to filter 82and the crystals are then dried on dryer 83.

In a specific operation the apparatus described above was operated usinga sodium sulfate decahydrate feed. This was fed into mixing tank 53 atthe rate of 1140 pounds per hour. Oil fuel was used in burner 43 and ata rate of 6.8 gallons per hour thus releasing 965,000 B. t. u. per hour.

-lVl'ateri-al was returned from the mixer 58 to the upper portion of theevaporator through pump Solution from the settling tank was supplied tothe sprays at 165 F. through-pump 67 at the rate of 35 gallons perminute. The temperature of the saturated gases to the blower 49 was 160F. and these gaseswere exhausted at the rate of 680 C. F. M. These gasesanalyzed ti /2% CO2 on a dry basis. Water was evaporated at the rate of628 pounds per hour in the process and anhydrou's'sodium sulfate wasproduced at the rate of 512 pounds per hour from dryer 83.

Since there is no heat transfer through a wall, it is evident that thereis no tendency for caking, scale or film formation. The rate of feed ofthe hydrated salt as made up is readily determined for any given set ofconditions and apparatus, it being only necessary to maintain the feedrate such that the liquid level in the thickener remains at the sameelevation, or increase amidecrease the heat input according to the feedrate.

The sodium sulfate decahydrate is fed into the mixing tank where itcomes in contact with the saturated solution from the evaporator. Someof the heat from the solution is absorbed in-melting and warming thefeed material. The sodium sulfate decahydrate is added at such a rate asto supply the water that is evaporated in the evaporating chamber, thusthere is no loss nor gain of circulating solution. The anhydrous sodiumsulfate crystals are formed in the crystallizer discharge temperature of130-185 F.

I have found that I have a measure of control of the crystal size bymanipulation of the discharge temperature of the crystallizer. Rathercoarse crystals of sodium sulfate are produced at 180 and become fineras the temperature is lowered to about where the product is mostly apowder. Our preferred operating range is with a crystallizer dischargetemperature of -185" F.

Since 'much more liquid is sprayed into the evaporating chamber than canbe evaporated, it is not diflicult to have a discharge gas that issaturated with water vapor. This makes for economical use of the fuelemployed. While the apparatus as shown is adapted to the use of oil as afuel it can easily be converted to use with any other type of fuel,solid, liquid or gaseous. Fuel efficiencies of 95% and better can easilybe attained with use of this apparatus. It will be evident that the flowsheet involves simple apparatus, and I have found that overall economyof operation is such as to enable us to produce anhydrous sodium sulfateat a cost less than that of otherprocesses in present use. Heats ofcrystallization, solution, vaporization and solubilities of saltsinvolved are the same using the method and apparatus herein described aswith equipment and methods commonly employed and are, therefore, omittedfrom thisdescription. These values can be found in any good textbook onthe subject.

While apparatus has been described-in connection with the dehydrationof;salts-"having an:

inverted solubility curve, and particularly sodium sulfate decahydrate,it will be evident that my invention is not so limited but that othersalts, including those having normal solubility curves, may likewise bedehydrated to advantage by my process and by making use of my apparatus.I conceive my invention to be broadly directed to the concentration ofsolutions of salts in water.

This is a division of application Serial No. 129,393, filed November 25,1949.

I claim:

1. A process for evaporating water from an aqueous solution of sodiumsulfate comprising continuously spraying the'solution into an upperportion of an evaporation zone, continuously introducing into an upperportion of said zone, but out of direct contact therewith, a dry hot gasand additional sodium sulfate solution, conducting said gas andadditional sodium sulfate solution downwardly through said evaporationzone but still out of direct contact therewith and then reversing thegas flow up into the evaporation zone into direct contact with saidsprayed solution, while confining said gas in a region bounded anddefined by a continuous film of solution, maintaining all surfacesdefining said zone wet continuously with a film of said solution,collecting solution from said zone adjacent the bottom thereof, andrecovering solid sodium sulfate from the solution,

'up through said evaporation zone and into direct contact with saidsecond portion of solution, maintaining all surfaces defining said zoneand in contact with said gas wet continuously with a film of saidsolution, collecting solution from said zone adjacent the bottomthereof, and recovering solid sodium sulfate from the solution.

JAMES V. WISEMAN.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 229,090 Burgess June 22, 1880 509,749 Morrell Nov. 28, 18932,090,984 Peebles Aug. 24, 1937 2,327,039 Heath Aug. 17, 1943 2,504,097Wiseman et a1 Apr. 11, 1950

1. A PROCESS FOR EVAPORATING WATER FROM AN AQUEOUS SOLUTION OF SODIUMSULFATE COMPRISING CONTINUOUSLY SPRAYING THE SOLUTION INTO AN UPPERPORTION OF AN EVAPORATION ZONE, CONTINUOUSLY INTRODUCING INTO AN UPPERPORTION OF SAID ZONE, BUT OUT OF DIRECT CONTACT THEREWITH, A DRY HOT GASAND ADDITIONAL SODIUM SULFATE SOLUTION, CONDUCTING SAID GAS ANDADDITIONAL SODIUM SULFATE SOLUTION DOWNWARDLY THROUGH SAID EVAPORATIONZONE BUT STILL OUT OF DIRECT CONTACT THEREWITH AND THEN REVERSING THEGAS FLOW UP INTO THE EVAPORATION ZONE INTO DIRECT CONTACT WITH SAIDSPRAYED SOLUTION, WHILE CONFINING SAID GAS IN A REGION BOUNDED ANDDEFINED BY A CONTINUOUS FILM OF SOLUTION, MAINTAINING ALL SURFACESDEFINING SAID ZONE WET CONTINUOUSLY WITH A FILM OF SAID SOLUTION,COLLECTING SOLUTION FROM SAID ZONE ADJACENT THE BOTTOM THEREOF, ANDRECOVERING SOLID SODIUM SULFATE FROM THE SOLUTION.